It is well known that a diesel engine requires the application of heat to certain parts of the engine prior to starting in order for the engine to start. The preheat elements used to preheat the engine are subject to burnout if used unnecessarily. Furthermore, if an engine is warm it is usually not necessary to preheat it. Current diesel engine preheat systems do not discriminate between warm and cold engines and a preheat switch is provided so that an operator may manually control the preheat according to his best judgement. If the operator's judgement is poor, this may lead to attempted starting before the engine is warm enough or, alternatively, it may lead to premature failure of the preheat elements because the preheating is carried out longer than required.